Ring modulators utilizing diodes have been used in the past to provide digital phase modulation of a data stream. An example of such a modulator is illustrated in U.S. Pat. No. 2,727,210 issued Dec. 13, 1955, and invented by D. G. Holloway. An inherent problem with a conventional diode ring modulator is that non-linearity in the diodes causes asymmetry of the switching envelope. In addition, the asymmetry and switching time of the diodes are not easily controlled independently of each other. Distortion due to this asymmetry can be minimized by fast switching of the diodes. However, the fast switching transfers additional power into higher order sidebands, which include fold-over components centered about zero frequency that appear in the passband at the output of the modulator. In addition, fast risetimes of the switching voltages are difficult to generate and control. A recent article which further discusses some of the problems encountered in such modulators is entitled: "Sources of Intermodulation In Diode-Ring Mixers" by H. P. Walker; The Radio and Electronic Engineer, May 1976, pp.247-255. Still another which compares the various types of modulators is entitled: "Comparison of Transformerless Ring Modulators and Cross Modulators" by Zdenek Mack; The Radio and Electronic Engineer, August 1974, pp.407-413.
An alternate form of modulator to the conventional ring modulator is a split-ring modulator. Examples of such modulators are found in U.S. Pat. No. 2,922,960 issued Jan. 26, 1960, and invented by B. R. Stachiewicz; and U.S. Pat. No. 2,958,051, issued Oct. 25, 1960, and invented by W. C. Perkins. These modulators may include steering diodes and a bias source to control the switching time. With this arrangement, symmetry is maintained independently of the switching time. The "switching time" refers to the time taken by the modulator to accomplish a 180.degree. phase reversal of the carrier to essentially the same constant output amplitude.
As is well known, phase modulation of a carrier produces a main power lobe centered about the carrier frequency as well as a plurality of ever-decreasing side lobes with nulls occurring at multiples of the data switching rate. The lower side lobes fold over about zero frequency with the result that spurious in-band side lobes will be generated in the passband of the modulator. This can be particularly disturbing in wide-band systems where the modulated carrier is only several times that of the data stream rate.
The problem of side lobe generation is discussed in an article entitled: "Spectrum Control in a Four-Phase PM Modem" by R. J. Tarry; Telecommunications, April 1968, pp.27-30. The solution proposed by this author, for suppressing the spurious in-band side lobes, is to modulate the carrier at a higher frequency, filter out the lower side bands and then translate the spectrum down in frequency. However, the example given is for phase modulation of a single voice channel and may not be practical for very wide band systems.